The various properties of carbon nanotubes, such as, for example, their stability, inertness, and relatively high surface area-to-volume ratio, make them effective carriers for drug delivery. The application of carbon nanotubes in drug delivery was first studied after the discovery that carbon nanotubes could penetrate into cells. Since this discovery, several in vitro studies have shown that carbon nanotubes can transport drugs, peptides, and proteins into cells. The shapes and sizes of carbon nanotubes are critical factors that affect the efficiency of drug delivery systems.
However, the high hydrophobicity, low functionality, and large size of pristine carbon nanotubes limit their biological applications. As a result, carbon nanotubes have been modified through the covalent or noncovalent functionalization of their external walls to enable the linkage of a variety of active molecules to the walls of carbon nanotubes. Nevertheless, functionalized carbon nanotubes have not been effective carriers of small molecules, such as antiviral, antibacterial, and anticancer agents.